In the past, a proposal has been made of an IGBT with a body diode that has diode elements and IGBT elements formed in the same semiconductor substrate (refer to, for example, a patent document 1, that is, IP-A-6-351226 corresponding to U.S. Pat. No. 5,559,656). The IGBT with a body diode has the anode electrodes of the diode elements and the emitter electrodes of the IBGT elements formed in common with each other, and has the cathode electrodes of the diode elements and the collector electrodes of the IGBT elements formed in common with each other. The IGBT with a body diode is incorporated in, for example, an inverter circuit, and used to control a load according to a pulse-width modulation (PWM) control method.
However, when the conventional IGBT with a body diode is incorporated in an inverter circuit, a gate signal for the IGBT elements is, in principle, a signal that has the phase thereof inverted between the upper and lower arms of the inverter circuit. The gate signal is therefore inputted to the IGBT elements even at the timing at which, for example, the diode elements freewheel. In other words, the action of the diode elements and the action of the IGBT elements take place simultaneously. Incidentally, the action of the IGBT elements signifies that the gate signal is inputted to the IGBT elements.
As mentioned above, when the action of the diode elements and the action of the IGBT elements take place simultaneously, since the electrodes are formed in common, if the channels in the IGBT elements conduct, the anodes and cathodes of the diode elements are brought to the same potential. Consequently, the body diode including the diode elements cannot readily act in a forward direction due to the gate potential of the IGBT elements. As a result, the forward voltage Vf of the diode elements increases, and the forward loss caused by the diode elements increases.
As a method for avoiding the foregoing problem by devising a device structure, formation of a diode-only region, that is, a region devoid of a gate separately from a body diode of an IGBT is conceivable as described in, for example, “Proceedings of 2004 International Symposium on Power Semiconductor Devices & Amp; ICs” (pp 261-264). However, a region that does not act as the IGBT, that is, a region that performs a diode action alone expands. Consequently, if the diode-only region is formed with a chip size left unchanged, the on-state voltage of the IGBT increases. Incidentally, if the on voltage of the diode is fixed, the chip size increases.
On the other hand, for dc-dc converters, a method of implementing synchronous rectification control by incorporating a double-diffused metal-oxide FET semiconductor (DMOS) with a body diode as a switching device in a control circuit is widely known. When a current flows into diode elements included in the DMOS with a body diode, a forward voltage is developed in the diode elements and a dc loss equivalent to the forward voltage is produced. Therefore, when such synchronous rectification control is implemented, a method of sensing the current in DMOS elements using a current transformer for the purpose of bringing a gate signal for reflux DMOS elements to an on-state voltage level is generally adopted (refer to, for example, JP-A-2004-180386).
However, the current transformer is needed as a current sensing device. This poses a problem in that the circuit scale gets larger. As a method that solves the problem, a method of monitoring a voltage across the terminals of a switching device is conceivable (refer to, for example, JP-A-2004-208407). However, according to this method, a control IC whose input terminal can withstand a high supply voltage is needed. Since noise resistivity is strictly requested at the time of conducting a high voltage, addition of a protective device or any other highly resistive design is needed. This poses a problem in that the cost of the control IC increases.
Thus, it is required to prevent an increase in a forward loss caused by a diode, which is included in a semiconductor device including an IGBT with a body diode, by avoiding the interference of the action of diode elements with the action of IGBT elements. Further, it is required to prevent an increase in a loss in a forward voltage of the diode elements, which are included in a semiconductor device including a DMOS with a body diode, by synchronizing the action of the diode elements with the action of DMOS elements.
As described in, for example, JP-A-2004-88001, a method of using current detection elements, which have the same structure as insulated-gate bipolar transistor (IGBT) elements do, to detect whether a current has flowed into freewheeling diode (FWD) elements, feeding back the result of the detection to a gate drive circuit, and setting the gate driving signal for the IGBT elements to an off-state voltage level when the FWD elements are put into action is conceivable. However, since the current detection elements having such a structure are affected by a gate potential, a current cannot readily flow into the current detection elements. A detective voltage cannot therefore be developed at the current detection elements. In other words, feedback cannot be precisely performed, and an increase in a forward loss caused by the commutation diode elements cannot be effectively suppressed.
Thus, it is required to provide a semiconductor device capable of suppressing an increase in a forward loss caused by FWD elements despite a construction having the FWD elements incorporated in IGBT elements.